Wednesday, September 14, 2016

An international team of astronomers
using Hubble have been able to study stellar evolution in real time.
Over a period of 30 years dramatic increases in the temperature of the
star SAO 244567 have been observed. Now the star is cooling again,
having been reborn into an earlier phase of stellar evolution. This
makes it the first reborn star to have been observed during both the
heating and cooling stages of rebirth.

Even though the Universe is constantly changing, most processes are
too slow to be observed within a human lifespan. But now an
international team of astronomers have observed an exception to this
rule. “SAO 244567 is one of the rare examples of a star that allows us to witness stellar evolution in real time”, explains Nicole Reindl from the University of Leicester, UK, lead author of the study. “Over
only twenty years the star has doubled its temperature and it was
possible to watch the star ionising its previously ejected envelope,
which is now known as the Stingray Nebula.”

SAO 244567, 2700 light-years from Earth, is the central star of the Stingray Nebula
and has been visibly evolving between observations made over the last
45 years. Between 1971 and 2002 the surface temperature of the star
skyrocketed by almost 40 000 degrees Celsius. Now new observations made
with the Cosmic Origins Spectrograph (COS) on the NASA/ESA Hubble Space Telescope have revealed that SAO 244567 has started to cool and expand.

This is unusual, though not unheard-of [1],
and the rapid heating could easily be explained if one assumed that SAO
244567 had an initial mass of 3 to 4 times the mass of the Sun.
However, the data show that SAO 244567 must have had an original mass
similar to that of our Sun. Such low-mass stars usually evolve on much
longer timescales, so the rapid heating has been a mystery for decades.

Back in 2014 Reindl and her team proposed a theory that resolved the
issue of both SAO 244567’s rapid increase in temperature as well as the
low mass of the star. They suggested that the heating was due to what is
known as a helium-shell flash event: a brief ignition of helium outside
the stellar core [2].

This theory has very clear implications for SAO 244567’s future: if
it has indeed experienced such a flash, then this would force the
central star to begin to expand and cool again — it would return back to
the previous phase of its evolution. This is exactly what the new
observations confirmed. As Reindl explains: “The release of nuclear
energy by the flash forces the already very compact star to expand back
to giant dimensions — the born-again scenario.”

It is not the only example of such a star, but it is the first time
ever that a star has been observed during both the heating and cooling
stages of such a transformation.

Yet no current stellar evolutionary models can fully explain SAO 244567’s behaviour. As Reindl elaborates: “We
need refined calculations to explain some still mysterious details in
the behaviour of SAO 244567. These could not only help us to better
understand the star itself but could also provide a deeper insight in
the evolution of central stars of planetary nebulae.”

Until astronomers develop more refined models for the life cycles of
stars, aspects of SAO 244567’s evolution will remain a mystery.

Notes

[1] The other star thought to have experienced the same type of helium flash event (see [2]) isFG Sagittae, located in the constellation Sagitta,
making SAO 244567 the second of its kind. However, other objects
undergoing similar “born-again” scenarios are known, includingSakurai’s Object, located inSagittarius.

[2] Helium flash events, also known as
late thermal pulses, occur late in the evolution of about 25% of low-
to medium-mass stars. After evolving off the main sequence, these stars
enter the red giant phase, where the star expands dramatically. Various
changes occur in the star’s chemical and physical composition during
this phase, until it has burnt most of the helium available in its core,
which is by then composed of carbon and oxygen. Helium fusion continues
in a thin shell around the core, but then turns off as the helium
becomes depleted. This allows hydrogen fusion to start in a layer above
the helium layer. After enough additional helium accumulates, helium
fusion is reignited, leading to a thermal pulse which eventually causes
the star to expand, cool and brighten temporarily.

More Information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

The results will be presented in the paper “Breaking news from the HST: The central star of the Stingray Nebula is now returning towards the AGB”, published in the Monthly Notices of the Royal Astronomical Society (MNRAS).